From German Patent Application No. B 422 II/20f, and from German Pats. Nos. 25 46 470 and 27 10 585, electro mechanical brakes are known, especially for rail vehicles, in which the brake request signal is led to an electrical actuator, which controls--eventually through a drive--the tension or the release of a spring loaded mechanism, the force of which produces the braking power to be led to the mechanical friction brakes. For this type of brake, German Pat. No. P 34 23 509 (published after the priority date of the present application) proposes for emergencies the provision of an emergency braking system, which uncouples the electrical actuator from the spring loaded mechanism in case of power failure, so that the spring loaded mechanism can deliver its full power as braking power. As this can easily lead, especially with an empty vehicle, to overbraking with blocking vehicle wheels, the patent application proposes a further mechanical limiting device for the deliverable braking power, which, by means of a mechanical vehicle load signal, controlled through a Bowden cable, limits the release of the spring load mechanism and thus the braking power. The setting, transmition and evaluation of the mechanical vehicle load signal is expensive, complicated and mostly relatively imprecise, and it has to be considered that in most brakes of this type the electrical brake demand signal already contains an electrical signal value for the vehicle load. However, it has to be taken into account that in emergencies, especially during power failure, the brake demand signal with its vehicle load dependent electrical signal value may completely fail, which may have the effect that, without the above-mentioned mechanical limiting device, braking at the maximum braking power that the spring loaded device can deliver may result, and thence overbraking as mentioned above.
According to the characteristics of the preamble of patent claim 1, the invention is based on brakes of the type described above.
Furthermore, electrically controllable pressure actuated brakes are known, which can particularly also constitute hydraulic brakes, wherein the control device in dependence on the brake demand signal produces a braking pressure which acts on a braking cylinder, and wherein the limiting device is a pressure limiting valve which limits the braking pressure to be led to the braking cylinder by the control device or the emergency braking device in dependence on the vehicle load. This type of pressure activated brake is known, e.g., from U.S. Pat. Nos. 3,730,597 and 3,920,285 and German Published Patent Application No. 28 01 778.9. Especially U.S. Pat. No. 3,920,285 shows clearly that a mechanical vehicle load signal, given off by the vehicle suspension, such as the pressure in the cushion-type pneumatic springs of the vehicle, is, on the one hand, converted into an electrical signal value which, together with the braking demand signal, is led to the control device for establishing a braking pressure, and that, on the other hand, an additional pressure limiting valve must be provided which is directly controlled by the mechanical vehicle load signal and, especially in cases of emergency upon loss of the mentioned electrical signal value or upon loss of all the electronics, effects a vehicle load dependent limit on the braking pressure for the prevention of overbraking. It is understood that arrangements such as in German Published Patent Application No. 28 40 262 can be equipped with this type of pressure control valves for the vehicle load dependent limitation of the brake pressure in case of failure of the electronic system. In the case of all these pressure actuated brakes, it is necessary to supply a mechanical vehicle load signal to the pressure limiting valve, the creation and supply of this often imprecise mechanical vehicle load signal being, as already mentioned, difficult and expensive, particularly as the pressure actuated brakes are already processing an additional electrical vehicle load dependent signal value.
From German Pat. No. 496 208 and U.S. Pat. No. Re 28,538 (re-issue of U.S. Pat. No. 3,547,499) it is already known to establish solely an electrical vehicle load signal for other types of pressure actuated brakes, and to evaluate it in the pressure actuated braking system for the creation of braking power. However, during power failure and the consequent emergency braking taking place for safety reasons, the electrical vehicle load signal gets lost, which might cause the already mentioned overbraking.